Source : Webster's Revised Unabridged Dictionary (1913)

Whitefish \White"fish`\, n. (Zo["o]l.)
   (a) Any one of several species of {Coregonus}, a genus of
       excellent food fishes allied to the salmons. They inhabit
       the lakes of the colder parts of North America, Asia, and
       Europe. The largest and most important American species
       ({C. clupeiformis}) is abundant in the Great Lakes, and
       in other lakes farther north. Called also {lake
       whitefish}, and {Oswego bass}.
   (b) The menhaden.
   (c) The beluga, or white whale.

   Note: Various other fishes are locally called whitefish, as
         the silver salmon, the whiting
   (a), the yellowtail, and the young of the bluefish
       ({Pomatomus saltatrix}).

Source : WordNet®

whitefish
     n 1: any market fish--edible saltwater fish or shellfish--except
          herring
     2: flesh of salmon- or trout-like cold-water fish of cold lakes
        of the northern hemisphere
     3: silvery herring-like freshwater food fish of cold lakes of
        the northern hemisphere
     [also: {whitefishes} (pl)]